In recent years, metal oxide having semiconductor characteristics which is referred to as an oxide semiconductor has attracted attention as a novel semiconductor material which has both high mobility, which is a characteristic of polysilicon, and uniform element characteristics, which is a characteristic of amorphous silicon. Examples of the metal oxide having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like.
Patent Documents 1 and 2 have proposed a thin film transistor in which metal oxide having semiconductor characteristics is used for a channel formation region.